Abstract 167: Ibrutinib regulates tumor microenvironment and enhances response to everolimus in renal cell carcinoma mouse models

Introduction: Ibrutinib (ibr), a first-in-class, once-daily, oral inhibitor of Bruton’s tyrosine kinase (BTK), is indicated for the treatment of patients with CLL/SLL, MCL and WM. Ibr also inhibits EGFR/HER2 and has demonstrated efficacy against EGFR+ NSCLC and HER2+ breast cancer in vitro and in xenograft models (Gao 2014; Chen 2016). Further, ibr modulated host immunity and enhanced anti-PD-L1 activity in solid tumor models otherwise insensitive to BTK or HER kinase inhibition (Sagiv-Barfi 2015), suggesting that ibr may be active in renal cell carcinoma (RCC) via multiple mechanisms. Here we determined the impact of ibr alone and in combination with everolimus (eve) on tumor growth and the tumor microenvironment in syngeneic and xenograft RCC mouse models. Methods: Cell proliferation was determined with CellTiter-Glo using a 3-day treatment regimen. Subcutaneous implantation of 786-0 and Renca cells established xenograft and syngeneic models in nude and immune-competent mice. Treatment started when tumor volume reached 150 mm3 (786-0) or 55 mm3 (Renca) and was measured twice weekly. Immunophenotying of tumor infiltrates was determined by flow cytometry. For Treg differentiation, magnetically-sorted CD4+CD25- mouse splenic T cells were stimulated with anti-CD3/CD28 antibodies and TGF-β1 for 5 days. Results: Ibr significantly inhibited the growth of syngeneic Renca tumors (TGI%=32.4 at Day 19, p≤0.05) in Balb/c mice and enhanced the efficacy of eve, although Renca cells were not sensitive to ibr in vitro. The combination significantly reduced the numbers of splenic and tumor Tregs in addition to reducing PD-1 expression on tumor CD8+ T cells. Consistently, in vitro Treg differentiation assays revealed that, while the combination of ibr and eve significantly reduced Treg differentiation, either drug alone showed little effect. In 786-0 xenograft model, ibr alone slowed tumor growth in a subgroup of mice and significantly enhanced the effect of eve (p≤0.05 or 0.01). To identify whether BTK or EGFR/HER2 is the target, CGI-1746, a BTK inhibitor, and lapatinib, an EGFR/HER2 inhibitor, were tested. Surprisingly, both CGI-1746 and lapatinib were active in inhibiting tumor growth (TGI%=26.7 or 37.6 respectively, p≤0.05 or 0.01), and slightly potentiated the efficacy of eve. However, consistent with ibr results, neither CGI-1746 nor lapatinib showed any effect on the cell growth of 786-0 cells. Conclusion: This study suggests that ibr has antitumor activity against